Composite oxide powder

A composite oxide powder comprising:

• a composition represented by a composition formula (1) below,
• wherein a ratio α/β of a specific surface area value α (m2/g) calculated by a BET one-point method to a specific surface area value β (m2/g) calculated from a formula (2) below is greater than 1.0 and equal to or less than 1.5,
• the specific surface area value α is equal to or less than 20 m2/g and
• the specific surface area value β is equal to or greater than 1.5 m2/g:
• ABO3-δ (1)
• wherein in the composition formula (1), A is one or more types of elements selected from the group consisting of La, Sr, Sm, Ca and Ba, B is one or more types of elements selected from the group consisting of Fe, Co, Ni and Mn and 0≤δ<1);
• and
• specific surface area value β (m2/g)=specific surface area value γ−specific surface area value ε (2)
• wherein in the formula (2), the specific surface area value γ (m2/g) is a cumulative value of specific surface area values in a total pore size range measured by a mercury intrusion method, and the specific surface area value ε (m2/g) is a cumulative value of specific surface area values in a pore size range of pore sizes that are larger than a 50% cumulative particle size (volume conversion-D50) in a particle size distribution calculated by a Microtrac particle size distribution measuring device.